Hi all! I got serious problem with the NFS server after binary upgrade to 4.9, I just can not store and retrieve files over NFS instead the client gets repeating message "nfs server not responding". More strange is that I can mount, unmount, list directory and even view files. I have no good idea how to fix or even debug the problem, as I don't get any messages or logs on the server. Appreciate any hints!

What release did you update from? If from 4.7 or earlier, did you upgrade in sequence, following the upgrade guide for each release? Did you also merge /etc and /var changes using sysmerge(8) or manually?

I suspect PF changes are the root cause, but that is just conjecture. If PF is used, disable it and see if you are able to connect.

NFS can be used safely, and in production environments, with proper planning and due care. This has been the case for decades. Let us find out more from warudemaru about what has been done or not done before casting aspersions upon a technology that may be providing significant value.

Thanks for your interest. My details are as follow: I did a normal upgrade from 4.8 using sysmerge according to the upgrade guide. On 4.8 NFS was running without problems, just like on previous versions. PF is used on that machine, but I don't believe it's a problem as I access it over home LAN. No changes were made to the pf.conf. However there were some changes to the sysctl.conf. Previously I had the optionsnet.inet.tcp.recvspace=131072
net.inet.tcp.sendspace=131072
net.inet.udp.recvspace=131072
net.inet.udp.sendspace=131072
Without these the network was struggling, the transfer rates were very low on 4.8. Now after upgrade to 4.9 net.inet.tcp just disappeared,but i'm not facing any troubles with speed rate anymore. Maybe some other tuning regarding NFS is now needed too?
Today I made some additional tests and got the following results: mount and umount, as well as ls and even editing a file is possible as long as I don't try to copy or move files after which the client process hangs in nfsreq state and any further NFS operations are impossible, only reboot helps.

Does the server have multiple network routes to the client? I'm guessing, of course, because you have not drawn us any diagrams.

You might try mount_nfs with -T ... If that functions better for you look to a network topology root cause.

I'd also be interested in the rest of your custom sysctls, and ask if you've used the diagnostic tools described in FAQ 6.7.

The topology is the most simple at all - OpenBSD as a router and clients behind, all on the same network, connected to one and the same switch.
I figured out that from 4.9 on I don't need any network options tweaking in sysctl.conf, so there is only net.inet.ip.forwarding=1 in there.
The problem with diagnostic tools is that they don't reveal much.. rpcinfo shows all services are runnig fine (they must as I can mount and ls), nfsstat shows some error probablyServer Ret-Failed 10Server Faults 0
NFS over TCP seems to help only that much it doesn't hang the client and the directory can be unmounted, but I believe that depends more on the client